Field
The present disclosure generally relates to the design of photonic integrated circuits (PICs). More specifically, the present disclosure relates to a PIC that includes an optical modulator based on a Mach-Zehnder interferometer (MZI) that uses light slowed by a lattice-shifted photonic crystal optical waveguide to enhance a semiconductor-oxide-semiconductor diode.
Related Art
Optical interconnects or links based on silicon photonics have the potential to alleviate inter-chip communication bottlenecks in high-performance computing systems that include a large number of processor chips and memory chips. This is because, relative to electrical interconnects, optical interconnects offer significantly improved: bandwidth, density, power consumption, latency, and range.
A high-speed optical modulator is an important component in a silicon photonic link. The function of an optical modulator is to convert a high-speed electrical data signal into optical form. The basic requirements of such an optical modulator are: high speed, low power consumption, low optical loss, a high on/off extinction ratio (ER), and compact size. To date, most of the reported high-speed optical modulators implemented using silicon are based on the free-carrier plasma dispersion effect, i.e., the index of refraction of silicon decreases as densities of electrons and holes (i.e., free carriers) increase. In order to use the free-carrier plasma dispersion effect for data modulation, the carrier densities in an optical waveguide need to be modulated. Thus, the index of refraction is modulated, and the optical phase of propagating laser light is modulated. As shown in FIGS. 1 and 2, the phase modulation can then be converted into optical intensity modulation (i.e., on/off switching) by building the phase-modulation optical waveguide into a ring-resonator modulator or a Mach-Zehnder interferometer (MZI).
Ring-resonator modulators typically use strong resonances for modulation. Consequently, they can achieve a large ER even with weak phase modulation. However, ring-resonator modulators usually require precise and dynamic tuning to align the resonance with the laser wavelength. This tuning can consume a large amount of electrical power and may require a large area for control circuits, which can significantly increase the cost of the ring-resonator modulators. Alternatively, MZIs usually do not require precise and dynamic tuning. However, they usually need much stronger phase modulation in order to achieve a large ER. An ideal modulator would be an MZI having a short length with strong phase modulation in the optical waveguide.
A variety of techniques are currently used to electrically modulate the carrier densities in the phase-modulation optical waveguides, including: carrier injection, carrier depletion and carrier accumulation. In carrier-injection phase modulation, high densities of free carriers are injected into the intrinsic region of a forward-biased PIN diode using a relatively small voltage (approximately, 1V). While this modulation technique is very efficient, its speed is typically limited to around 1 Gb/s by minority-carrier diffusion.
In carrier-depletion phase modulation, the carrier-depletion region of a reverse-biased PN diode is modulated. Because this modulation technique does not involve minority-carrier diffusion, it can be very fast. However, carrier-depletion modulation is often inefficient because it is hard to deplete a lot of charge. Consequently, for efficient modulation, carrier-depletion modulation is typically implemented in a ring-resonator modulator or an MZI having a long length. Therefore, phase modulation based on carrier injection or carrier depletion usually cannot simultaneously produce a high-speed (greater than 10 Gb/s), very short MZI (less than 0.3 mm) with a reasonably large ER (greater than 5 dB) under small voltage modulation (less than 2V).
Carrier-accumulation phase modulation attempts to combine the advantages of carrier-injection phase modulation and carrier-depletion phase modulation. In this technique, a diode with a forward-biased PN junction having a very thin (less than 10 nm) oxide barrier layer (which prevents minority-carrier diffusion) is used. Under forward-biased voltage, the diode operates in an accumulation mode, in which large densities of carriers accumulate at the two sides of the oxide layer. This approach is usually much more efficient than carrier-depletion phase modulation, but is typically less efficient than carrier-injection phase modulation. Using carrier-accumulation phase modulation, it is possible to build a short MZI (around 0.5 mm) with high carrier-mobility and low voltage operation. However, the diode typically has much higher optical loss (greater than 100 dB/cm) than the PN diodes (less than 10 dB/cm) because of polycrystalline-silicon scattering and absorption. Consequently, the resulting modulator may have higher optical loss even though its length is much shorter (approximately 6×). In addition, the diode typically has a very high unit capacitance, which prohibits traveling-wave design and limits modulation speed (because of RC limits associated with the MZI and driver). In order to achieve greater than 10 Gb/s with a lumped-element design, it is important to lower the capacitance of the MZI to less than 0.5 pF, while maintaining high efficiency.
Hence, what is needed is an optical modulator without the problems described above.